Being able to change username

[nilsreichardt]

Description

Because of the issue in #3 I need to use my own person access token. But if this action merges a pull request, it always be display that I have merged this PR.

My goal is to change the username that is displayed. Normally, you can do this with:

git config user.name "github-actions"

But the run option is now available, if you use uses in a GitHub action job.

Would be an env option, like USERNAME a good solution?